1. Field of the Invention
This invention relates to dental burrs and dental handpieces for rotatably holding such a dental burr.
2. Prior Art
FIG. 17 shows a conventional dental burr A and a conventional dental handpiece B both of which are of the general type, the dental handpiece being adapted to hold the dental burr A and drive the same for rotation about a longitudinal axis thereof to cut or bore the tooth of the patient.
The dental burr A comprises a shank portion A.sub.1 at its rear portion and a cutting portion A.sub.2 at its front portion, these two portions A.sub.1 and A.sub.2 having a common longitudinal axis or center line.
The handpiece B has a bar-like handgrip portion B.sub.1 and a hollow head portion B.sub.2 provided at a front end the handgrip portion B.sub.1 The head portion B.sub.2 contains therein a mechanism (not shown) for chucking the shank portion A.sub.1 of the dental burr A and imparting a rotational movement thereto. The handpiece B also includes a cooling water passage B.sub.3 which is formed in and extends along the handgrip portion B.sub.1, the cooling water passage B.sub.3 being connected at one end to a source of cooling water and opening at the other or distal end to a lower surface of the head portion B.sub.2. This conventional handpiece B is disclosed in Japanese Laid-open (Kokai) Utility Model Application No. 56-51845.
When the dental burr A supported by the dental handpiece B is rotated at a high speed to cut the tooth of the patient, heat is generated at an area of contact between the cutting portion A.sub.2 and the tooth. For this reason, during this cutting operation, the cooling water is injected toward the cutting portion A.sub.2 of the dental burr A from either the distal end of the cooling water passage B.sub.3 or a nozzle attached thereto to thereby cool the cutting portion A.sub.2 and the tooth.
With the above dental burr A and handpiece B, however, an adequate cooling effect can not be achieved. The reason is that the cooling water is not sufficiently supplied to the area of contact between the cutting portion A.sub.2 of the dental burr A and a surface defining a bore formed in the tooth by this cutting operation despite the fact that the heat is generated in this contact area.
The following disadvantages are caused by such insufficient cooling effect:
Firstly, the cutting portion A.sub.2 of the dental burr A is subjected to high temperatures during the cutting operation and therefore undergoes a premature wear.
Secondly, the heat generated during the cutting operation stimulates the nerve of the tooth or the nerve of the gingiva or gum. As a result, the patient feels a pain. More specifically, for example, in the case of the treatment of a slightly-decayed tooth of which nerve has not yet been extracted, the heat thus generated is transmitted to the nerve of the tooth though the dental burr A is not in direct contact with this nerve. Also, in the case where the decayed portion of the tooth is bored by the dental burr in the vicinity of its root after extracting the tooth nerve, the heat is transmitted to the nerve of the gingiva. In these cases, anesthesia must be used to relieve the pain of the patient although the use of anesthesia is not desirable.
FIG. 18 shows a dental burr A' disclosed in U.S. Pat. No. 3,393,452. The dental burr A' has a cooling water passage A.sub.3 formed therein and extending along its longitudinal axis. The cooling water passage A.sub.3 opens at one end to a rear end face of a rear shank portion A.sub.1 and terminates at the other end slightly short of a front end face of the cutting portion A.sub.2. The dental burr A' also has a plurality of radially-extending ports A.sub.4 formed through a peripheral wall of a front cutting portion A.sub.2 of the dental burr A'. More specifically, each of the radial ports A.sub.4 communicates at one end with the cooling water passage A.sub.3 while the other end of the port A.sub.4 opens to the outer peripheral surface of the cutting portion A.sub.2.
In the dental burr A' of FIG. 18, the cooling water is supplied from the cooling water passage A.sub.3 to the outer peripheral surface of the cutting portion A.sub.2 through the radial ports A.sub.4, and therefore the cooling water can be supplied to the area of contact between the outer peripheral surface of the cutting portion A.sub.2 and the surface defining a bore formed in the tooth by the cutting portion A.sub.2 to achieve an effective cooling operation.
However, the dental burr A' still has the following disadvantage:
When the dental burr A' is moved axially to deepen the bore being cut, the cooling water fails to be sufficiently supplied to the area of contact between the front end face of the cutting portion A.sub.4 and the bottom face of the bore despite the fact that the heat is generated particularly at this contact area. As a result, such heat is not efficiently removed. Therefore, the stimulation of the tooth nerve or the gingiva nerve can not be restrained sufficiently. In addition, the wear of the front end face of the cutting portion A.sub.2 can not duly be restrained.
Further, the above mentioned two dental burrs A and A' have a common drawback. More specifically, the speed of rotation of the front end face of the cutting portion A.sub.2 is very low in the vicinity of the axis of rotation thereof, and therefore that portion of the front end face in the vicinity of the axis of rotation performs substantially no cutting function Therefore, when the dental burr A, A' is moved axially to deepen the bore being cut, said that portion of the front end face of the cutting portion A.sub.2 is much resisted by the bottom face of the bore, so that a good cutting performance is not obtained.
As disclosed in Japanese Laid-Open (Kokai) Patent Application No. 58-65152, there is known prior to the present invention a handpiece which comprises a chuck mechanism for chucking a dental burr and a push button for releasing the chucking operation. As far as the inventor of the present invention knows, there is not known any handpiece of the type designed to supply the cooling water to such a dental burr provided with the cooling water passage as disclosed in the aforesaid U.S. Pat. No. 3,393,452.